Inductive position sensor for determining the angular position of a rotating shaft

ABSTRACT

The sensor comprises a field coil generating a time-variant magnetic field, and sensor coils arranged inside the field coil, a rotatable device being arranged for modulating the magnetic flux passing through the sensor coil according to its angular position. Each sensor coil comprises a magnetically permeable core having a polar portion facing the rotatable device. A portion of a magnetically permeable sealing element is arranged facing the rotatable device on the opposite side from said polar portion. The air gaps between said polar portion and rotatable device, and between said portion of the sealing element and said rotatable device, are wholly located substantially within the field coil.

FIELD OF THE INVENTION

The present invention relates to a position sensor for determining theangular position of a rotating shaft comprising a field coil forgenerating a time-variant magnetic field, at least one sensor coilarranged inside the field coil and a rotatable member arranged formodulating the magnetic flux passing through the sensor coil inaccordance with its angular position.

BACKGROUND OF THE INVENTION

Sensors of this type are known in the art. The angular position of ashaft is determined from the signal obtained at the terminals of thesensor coil or coils. However, in the known devices, the amplitude ofthe variation of the signal as a function of the angular variation ofthe shaft is relatively small and the precision of the measure is ofteninsufficient.

SUMMARY OF THE INVENTION

The position sensor according to the invention comprises a field coilfor generating a time-variant magnetic field, a magnetically conductivehousing structure supporting the field coil, at least one sensor coilarranged inside the field coil and supported by the housing structure,and a rotatable member supported for rotation inside the housingstructure. The rotatable member is coupled with said rotating shaft andarranged for modulating the magnetic flux passing inside the sensor coilin accordance with its angular position. Each sensor coil comprises awinding and a magnetically permeable core with a flat polar partextending outside said winding so as to face the rotatable member. Atleast one portion of said housing structure coupled with said field coilis arranged to face the rotatable member on the opposite side from saidpolar part. The arrangement is such that the air-gaps existing betweensaid polar part and said rotatable member on the one hand, and saidportion of said housing structure and the rotatable member on the otherhand, are located substantially within the field coil.

It is an object of the present invention to provide a position sensor ofthe type mentioned at the beginning, which has a simple, economic andcompact structure and which produces a position signal of a relativelyhigh amplitude and of very good precision.

The features and the advantages of the present invention will be betterunderstood in the light of the following description relating to oneexample of embodiment of the sensor according to the invention,illustrated by the attached drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial section of a position sensor according to theinvention, and

FIGS. 2(a) to 2(f) are a simplified representation illustrating theoperation of the sensor through different positions of the rotatablemember.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The sensor of FIG. 1 comprises a housing composed of a supporting plate11 and a closing member 12 made of a magnetically permeable material. Apart of plastic material 13 is arranged inside this housing andconstitutes, in particular, a coil body 14 and a bearing part 15 inwhich the end of a rotatable shaft 1 is lodged. The shaft 1 is integralwith a rotatable member comprising a disc-shaped support 4 and a part 5such as a ferrite tablet in the present example.

The coil body 14 is used as a support for the winding 2 of a cylindricalfield coil which extends over practically the whole height of theclosing member 12. On the supporting plate 11, four sensing coils arearranged in a symmetrical way, each being constituted by a winding 3, acoil body 16 and a core 6. Each core 6 is provided with a polar part 7extending above coil 3 so as to conduct the magnetic flux generated bythe field coil winding 2. Two of these sensing coils are shown in axialsection in FIG. 1.

In the present example, the polar parts 7 have a circular shape and theferrite tablet 5 is also substantially of circular cylindrical shape.The support 4 is preferably made of an electrically conductive material,for example of aluminum. The closing member 12 comprises a part 8 whichis arranged opposite the rotatable member and which forms with the samean air-gap 10, while an air-gap 9 is formed between the rotatable memberand the polar parts 7. These air-gaps have preferably a minimal heighthaving regard to the manufacturing tolerances which must allow themovement of the rotatable member. As will be noted from FIG. 1, not onlythe air-gap 9 but also the air-gap 10 is located substantially insidethe field coil 2, so that the magnetic field generated by a current inthis field coil is substantially uniform inside these air-gaps. Thisfield is closed essentially through the cores 6, the polar parts 7, thesupport plate 11 and the closing member 12.

Upon rotation of the shaft 1, the magnetic flux which passes through thesensor coils and which varies in time as the energizing current in coil2, varies in amplitude in each coil as a function of the angularposition of the tablet 5 with respect to the corresponding polar parts7.

FIGS. 2(a) to 2(f) illustrate different positions of the rotatablemember with respect to the polar parts of the four sensor coils whichare connected in a known way, so that two diametrically opposite coilsform half-phases of a same phase. The shape and the dimensions of thepolar parts and of the tablet 5 are preferably chosen in such a way thatthe variation of the amplitude of the magnetic flux is continuous in thetwo coils of a same phase, in other words, such that when the tablet 5no longer covers (region R) partially a polar part, it starts coveringthe diametrically opposite polar part. Different characteristics of fluxvariations as a function of the angular position of the rotatable membercan be obtained by the shapes and the dimensions of the facing parts,this variation being, for example, in particular sinusoidal.

The variation of the magnetic flux passing through the different coilsresults in a corresponding modulation of the voltage in the sensor coilswhich provides the measuring signal. The support 4 of the magneticallypermeable part 5 will, in the present case, be the seat of eddy currentswhich reduce the strength of the field outside part 5 and thus increasethe difference between the maximum and minimum values of the voltageinduced in the sensor coils. To avoid that these eddy currents alsoclose around the tablet 5, a gap can be provided in the support 4 at aplace on the periphery of that part, between the same and the edge ofsupport 4.

The present arrangement of the position sensor leads to a particularlycompact structure reduces the losses of magnetic energy and provides arelatively high output signal. Furthermore, the precision of such asensor is very good due to the uniformity of the magnetic field and therelative indifference to the manufacturing tolerances. It is to be notedthat the air-gaps 9 and 10 sum up and that their individual variationsin dimension have practically no influence on the measure. As FIG. 1shows, the volume of the sensor is used in an optimal way for lodgingthe greatest volume of active elements.

It is to be noted that, besides a flux modulation by a part such as 5which increases the flux in the region it overlaps, it is also possibleto provide, for example, one or more electrically conductive turns ofcoils to reduce, on the contrary, the field passing through that region.Other embodiments are also available within the scope of the presentinvention to a person skilled in the art.

I claim:
 1. A position sensor for determining the angular position of arotating shaft, comprising a cylindrical field coil for generating atime-variant magnetic field, a magnetically conductive housing structuresupporting said field coil, at least one sensor coil arranged insidesaid field coil and supported by said housing structure, and a rotatablemember supported for rotation inside said housing structure, saidrotatable member being coupled with said rotating shaft through anaperture in said housing structure and being arranged for modulating themagnetic flux produced by said time-variant magnetic field and passinginside the sensor coil in accordance with its angular position, eachsensor coil comprising a winding and a magnetically permeable core witha flat polar part extending outside said winding so as to face therotatable member, at least one portion of said housing structure coupledwith said field coil being arranged to face the rotatable member on theopposite side from said polar part, the arrangement being such that theair-gaps existing between said polar part and said rotatable member onthe one hand, and said portion of said housing structure and therotatable member on the other hand are located substantially within saidfield coil.
 2. Sensor according to claim 1, characterized in that theheight of said air-gaps has a minimum value having regard to themanufacturing tolerances.
 3. Sensor according to claim 2, characterizedin that the rotatable member comprises a part of magnetically permeablematerial in a non-magnetic support.
 4. Sensor according to claim 3,characterized in that the support is in an electrically conductivematerial.
 5. Sensor according to claim 4, characterized in that thesupport has an electric discontinuity at at least one place on theperiphery of the permeable part of the rotatable member.
 6. Sensoraccording to claim 1, characterized in that the rotatable membercomprises a part of magnetically permeable material in a non-magneticsupport.
 7. Sensor according to claim 6, characterized in that thesupport is in an electrically conductive material.
 8. Sensor accordingto claim 7, characterized in that the support has an electricdiscontinuity at at least one place of the periphery of the permeablepart of the rotatable member.